Conventionally, a so-called variable suction system, in which the suction runner length supplied to the combustion chamber is varied in response to the operating state of an engine, serves to lengthen the suction runner at a low speed and on a low load to increase the inertia force for an increased efficiency, and to shorten a suction runner at a high speed and on a high load to reduce the suction resistance for an increased efficiency.
In order to make the suction runner variable, the surge tank and the suction manifold are increased in size, and the surge tank and the suction manifold are formed in a compact external size, if possible, in terms of engine room layout.